1. Field of the Invention
This invention generally relates to a motion estimation device and the power saving method thereof, and more particularly, to a motion estimation device and the power saving method thereof both of which can save total power consumption by adjusting the emitting cycle of a light source of the motion estimation device.
2. Description of the Related Art
In order to increase the operational convenience of computer equipments, wireless optical mice have gradually replaced the traditional wired mouse. An optical mouse generally includes a plurality of power consuming components, such as a light source, a digital signal processor, an image sensor and a wireless transmission unit. It has high consumption power and therefore has the problem of limited battery lifetime. In order to solve this problem, the method to reduce the power consumption of an optical mouse by adjusting the image capture frame rate of an image sensor has been proposed in the prior art. For example, U.S. publication No. 20050110746, entitled “Power-saving method for an optical navigation device”, discloses a method for controlling the image capture frame rate of an image sensor according to a moving speed, i.e. variations of a horizontal displacement and a vertical displacement per unit time, of an optical navigation device, wherein when the moving speed is faster, the image capture frame rate becomes higher and the exposure time of the image sensor becomes shorter; on the contrary, when the moving speed is slower, the image capture frame rate becomes lower and the exposure time of the image sensor becomes longer. In this manner, the power of the optical navigation device can be saved.
However, the above method, which can save the power consumption of an optical navigation device by adjusting the image capture frame rate and the exposure time of an image sensor, still has following problems. (1). Since the exposure time of the image sensor changes with the moving speed of the optical navigation device, total image brightness appears larger fluctuation to have lower stability. (2). Although the image capture frame rate can decrease with the decreasing of the moving speed of the optical navigation device, the sensing area of the image sensor has to be increased so that the same maximum detectable acceleration obtained under low speed moving status can be equal to that obtained under high speed moving status. Therefore, the loading of the digital signal processor will not linearly decrease with the decreasing of the image capture frame rate.
Another power saving method for an optical navigation device is disclosed in U.S. Pat. No. 6,455,840, entitled “Predictive and pulsed illumination of a surface in a micro-texture navigation technique”. The method changes the illuminating rate of a light source according to the velocity of the optical navigation device so as to conserve power. The disclosed method includes the steps of: illuminating the surface with pulses of light occurring at a selected rate; determining a velocity from the tracked movement; increasing the selected rate in response to increases in the determined velocity; and decreasing the selected rate in response to decreases in the determined velocity.
However, the above velocity calculation of the optical navigation device is held based on the pixel size and the frame rate. Not only the signal processing load is increased due to an additional calculating procedure, but the velocity information (for example a velocity threshold), which is served as the reference for adjusting the illuminating rate of the light source, has to be designed differently for high-end and low-end products. For example, for high-end products, the velocity threshold is relatively higher; while for low-end products, the velocity threshold is relatively lower. Therefore, the above power saving method based on changing the illuminating rate of a light source according to the velocity still has following problems. (1). Signal processing load is increased to increase the total power consumption. (2). Different velocity information has to be designed for different products; that is, same velocity information can not be used for all products such that the product design complexity will be increased.
According to the above reasons, the present invention provides a method for dynamically adjusting the emitting cycle of the light source of a motion estimation device according to a displacement of the motion estimation device. When the displacement is relatively low, the emitting cycle of the light source is prolonged so as to decrease the power consumption of the light source. In this manner, the motion estimation device needs not to transfer the detected displacement to moving speed such that the signal processing load can be decreased so as to decrease the total power consumption.